1. Field of the Invention
The present invention relates to a battery charging method, and particularly to a battery charging method that can increase the maximum charging capacity of a battery and additionally can increase life of the battery.
2. Description of the Related Art
Recently, a battery charging method that charges a battery to a full charged state at a specified voltage is used to charge lithium-ion rechargeable batteries. In a lithium-ion rechargeable batteries rises, a charging process is conducted so that a lithium ion battery is charged initially at a constant current and is then charged at a constant voltage to a full charged state. The initial charging process part is conducted until a battery voltage reaches the specified voltage. After the battery voltage reaches the specified voltage, the process is changed to the constant voltage charging process part so that the battery voltage does not rise. The reason is to prevent that the voltage of a battery to be charged does not exceed the specified voltage. In the constant voltage charging process part in which the battery is charged with its voltage being held at the specified voltage, a battery charging current gradually decreases. When the charging current becomes smaller than a specified value, it is determined that the battery is fully charged, thus, the charging process is completed. In lithium-ion rechargeable batteries, their specified voltage is specified so that the voltage of a battery in the full-charged state becomes 4.1 V/cell to 4.2 V/cell, for example.
In the aforementioned method that charges a battery to the full charged state at a limited voltage, the degree of a specified voltage affects an actual capacity that actually discharges the batteries, and life of the battery. FIG. 1 is a graph showing the actual capacity and the cycle life of a lithium-ion rechargeable battery. In this graph, the horizontal axis shows the charging/discharging process cycle, and the vertical axis shows the actual capacity. This graph clearly shows in the case where the specified voltage is set high, the initial capacity of the battery can be high. On the other hand, if the specified voltage is set high so that the battery is fully charged to a high full-charged voltage, the life of the battery decreases. In FIG. 1, properties A, B, C and D show the properties of lithium-ion rechargeable batteries that are charged at different specified voltages. The property A battery is charged at a specified voltage that is set high. The property A battery has a high initial actual capacity, but its cycle life decreases. Conversely, the property B, C and D batteries are charged at specified voltages that decrease in this order, thus, their initial actual capacities decrease, but the length of their cycle life increases. These properties in this Figure show the actual capacity and the cycle life are mutually contradictory. If a specified voltage is set high so that an actual capacity increases, cycle life decreases. On the other hand, if a specified voltage is set low so that life cycle increases, an actual capacity decreases. Accordingly, one of actual capacity and cycle life is improved, the other deteriorates. Both of them cannot be satisfied.
To improve the life property of a battery, methods have been developed which charge a battery at a specified voltage that increases as charging/discharging processes are repeated (see Japanese Laid-Open Publications Nos. H09-120843, 2000-270491, and 2005-278334).